Adaptive confirmation of treatable arrhythmia in implantable cardiac stimulus devices

ABSTRACT

Methods and devices for adjusting therapy delivery decisions in an implantable cardiac stimulus device by observing cardiac activity following an initial identification of a treatable condition. In some examples, cardiac activity that appears benign is quantified and a therapy confirmation threshold is adjusted according to how much apparently benign cardiac activity is seen after an initial identification of a treatable condition. In other examples, a new threshold is applied following the initial identification of treatable condition, removing historical data preceding the initial identification from subsequent therapy delivery decisions.

RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 12/826,241, filed Jun. 29, 2010, which claims the benefits ofand priority to U.S. Provisional Patent Application No. 61/221,316,titled CONFIRMATION OF TREATABLE ARRHYTHMIA IN IMPLANTABLE CARDIACSTIMULUS DEVICES, filed 29 Jun. 2009, and the entire disclosure of whichis incorporated herein by reference.

FIELD

The present invention relates generally to implantable medical devicesystems that sense and analyze cardiac signals. More particularly, thepresent invention relates to implantable medical devices that analyzecardiac signals in order to classify cardiac activity as likely benignor treatable.

BACKGROUND

An implantable cardiac stimulus device (ICSD) typically senses cardiacelectrical signals in an implantee and uses the sensed signals toclassify the implantee's cardiac rhythm as normal/benign ortreatable/malignant. Illustrative treatable arrhythmias may includeventricular fibrillation and/or ventricular tachyarrhythmia. Otherrhythms may also be considered “treatable” depending upon patientcharacteristics and physician preferences.

If a treatable arrhythmia is identified and defibrillation orcardioversion is to be delivered, an ICSD typically needs some period oftime to prepare for therapy delivery. For example, a three, six ornine-volt battery supply may be used to provide hundreds or eventhousands of volts of stimulus amplitude by charging a capacitor forseveral seconds using a charging circuit. As a result, once a treatablecondition is identified, there is a charging time period correspondingto the preparations of the device to deliver therapy. Anti-tachycardiapacing (ATP) may not require such a charging delay, however, to ensurethat therapy is appropriate, a system delivering ATP may include a delayfor confirmation of arrhythmia before therapy delivery.

Some treatable arrhythmias may be intermittent or may spontaneouslyrevert to a benign rhythm. If an apparently treatable arrhythmiaspontaneously reverts to a benign rhythm, therapy becomes unnecessary.In general, therapy delivery should be managed to avoid unnecessarytherapies.

To avoid unnecessary therapy, devices may perform rhythm confirmationjust before delivering therapy. New or alternative methods for managingdelivery through therapy confirmation are desired.

SUMMARY

A first illustrative embodiment is a method of patient treatment usingan implantable cardiac stimulus device (ICSD). The first embodiment maybe performed in the context of a system that has made an initialidentification of a treatable condition. In the example, after theinitial identification of a treatable condition, cardiac activity ismonitored to identify indications of a reversion to benign cardiacrhythm while the ICSD prepares to deliver therapy. Once the ICSD isprepared to deliver therapy, analysis is performed to confirmwhether/when therapy should be delivered. If indications of a reversionto benign cardiac rhythm are identified, therapy confirmation analysisis adjusted to delay therapy delivery in order to avoid deliveringtherapy into a benign rhythm. If benign cardiac rhythm persists, thetherapy delivery may also be cancelled. In some examples, the ICSDprepares to deliver therapy by, for example, transferring data tomemory, recording sensed signals, establishing a wait period in terms oftime or detected events, identifying synchronization data and/or otherfunctions. In some embodiments and ICSD prepares for therapy delivery bycharging a therapy delivery capacitor to a desired energy/voltage,though this is not always part of the preparation.

In one example, initial treatable condition criteria is/are applied todetermine whether to advance to confirmation methods, and differentcriteria are applied for confirmation. In one illustrative example,intervals between detected events are analyzed to use rate as criteriafor initial identification and confirmation. In further examples,morphology and/or combinations of interval analysis and morphology maybe used to identify indications of benign or treatable arrhythmias. Insome examples, one of rate or morphology is used for initialidentification, and the other or a combination of rate and morphologycan be used for confirmation. Indications of benign conditions can betracked and quantified after the initial identification of a treatablecondition and are then used in the confirmation step to delay therapydelivery.

In another embodiment, a method of patient treatment in an ICSD appliesa set of criteria to determine whether a treatable condition appearslikely. Once an initial treatable condition is identified, the data thatled to the initial identification is cleared and confirmatory criteriaare applied. If the confirmatory criteria are met, therapy is delivered.In one such example, an X/Y counter is used to analyze a window of datafor the initial treatable condition identification, and the X/Y counteris cleared and re-filled for applying confirmatory criteria.

In another embodiment, an X/Y counter is used to generate an initialanalysis of treatable condition. Upon an initial finding of a treatablecondition, the values in the X/Y counter are stored, and analysiscontinues to fill the X/Y counter with new data. A confirmation step isthen performed when the status of the X/Y counter is compared to thestatus stored after the initial analysis. In the example, therapy willbe delivered once the X/Y counter reaches a status that is at leastequivalent to that which occurred at the time of initial analysis.

The ICSD may be configured to deliver therapy such as cardioversion ordefibrillation that calls for a charging time during which a therapycapacitor is charged to a desired level for therapy. If so, and inseveral examples, the device begins charging to prepare for therapydelivery upon identification of the initial treatable condition. Forsome such examples, the confirmation analysis may include checkingwhether preparation for therapy delivery is complete as well asanalyzing data sensed before, during or after charging began orfinished. The invention is not limited to analysis during/followinginitiation of capacitor charging, as, for example, some therapies (suchas ATP) may not require capacitor charging. For example, preparation fortherapy can follow the initial identification of treatable condition byincluding a delay period defined in terms of a time delay or a number ofdetected events; part of the “preparation” includes simply waiting toensure accuracy of the decision-making process.

Additional embodiments include devices and systems configured or adaptedto perform the above methods. The present invention can be embodied in alarge number of different examples, only a few of which are summarizedhere in this “Summary”. Further examples are shown below in the“Detailed Description.”

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows, in block form, an illustrative method for managingdetection results during and after charging in an ICSD;

FIG. 2 shows an illustrative implantable cardiac stimulus systemrelative to a patient's heart;

FIG. 3 shows, in block form, an illustrative method of operating anICSD;

FIG. 4 is a process flow diagram for an illustrative method for managingtherapy delivery;

FIG. 5 is an illustration of analysis using a “Last 3 intervals” therapyconfirmation criteria;

FIGS. 6A-6C illustrate analysis similar to FIG. 5, except with differenttherapy confirmation criteria applied;

FIGS. 7A-7B show a process flow for another illustrative method formanaging therapy delivery;

FIG. 8 illustrates analysis using the method of FIGS. 7A-7B; and

FIG. 9 is a block diagram for an illustrative method.

DETAILED DESCRIPTION

The following detailed description should be read with reference to thedrawings. The drawings, which are not necessarily to scale, depictillustrative embodiments and are not intended to limit the scope of theinvention. Some of the following examples and explanations includereferences to issued patents and pending patent applications. Thesereferences are for illustrative purposes and are not intended to limitthe present invention to the particular methods or structures from thereferenced patents and patent applications.

Unless implicitly required or explicitly stated, the methods below donot require any particular order of steps. It should be understood thatwhen the following examples refer to a “current event,” in someembodiments, this means the most recently detected cardiac event isbeing analyzed. However, this need not be the case, and some embodimentsperform analysis that is delayed by one or more detections and or afixed period of time. Choices shown regarding use of rectified orunrectified signals are merely illustrative, and may be changed ifdesired.

Several embodiments disclosed herein can be used in an implantablecardiac stimulus device (ICSD). One particular type of ICSD is animplantable cardioverter-defibrillator, which typically provides therapyin the form of cardioversion and/or defibrillation therapies and, whenneeded/programmed, anti-bradycardia pacing. An ICSD may provideanti-tachycardia pacing, for example in a tiered therapy system, as wellas other functionality including but not limited to other pacingtherapies or any other suitable therapy. An ICSD may also treat atrialarrhythmias.

FIG. 1 shows, in block form, an illustrative method for managingdetection, analysis and therapy delivery in an ICSD. Three generalstates of operation are shown—at the top, a Not Concerned state, in themiddle, a Initial Treatable Condition Found State, and at the bottom, aConfirmation state. The Not Concerned State implies that a treatablecondition, such as a ventricular or atrial arrhythmia has not beenidentified. The Initial Treatable Condition Found State indicates that atreatable condition has been identified and the ICSD is preparing todelivering therapy. The Confirmation state indicates that the ICSD isperforming analysis to confirm that therapy should be delivered.

In one example, an ICSD may be designed to wait for a period of time ora number of cardiac events or detected events before delivering therapyafter it identifies a treatable condition. In yet another example, anICSD may be designed to make a first, initial identification oftreatable condition, but waits for a reconfirmation of the treatablecondition after clearing out data used in the initial identification.For example, an ICSD may include a capacitor and a charging circuit, andthe Initial Treatable Condition Found state can be used while the ICSDcharges the capacitor to a desired therapy voltage. When the desiredtherapy voltage is reached, analysis confirms therapy should bedelivered in the Confirmation state. Once therapy is confirmed, the ICSDmay adjust the charge on the capacitor to ensure the correct outputenergy and delivers therapy. Capacitor charging and other conditions mayeach be applied in parallel by requiring, for example, that the ICSDwait at least a predefined period of time, as well as that the ICSD waituntil a capacitor is charged to a desired level and, once bothconditions are met, the device is ready to deliver therapy and canadvance to the Confirmation state.

The Not Concerned State shows detection loop 12, which applies criteriadesigned to detect cardiac events. One example compares a detectionprofile to sensed signal amplitude, using methods known in the art. Oncean event is detected, analysis 14 is called to determine whether adetected event or series of detected events indicate a treatablecondition, such as an arrhythmia. The specific method of detecting atreatable condition may vary. In one example, the analysis 14 uses eventrate as a first indicator of whether a treatable condition is occurring.For example, if the calculated event rate is high, a treatabletachyarrhythmia may be occurring. Morphology may be consideredincluding, for example, considerations such as correlation to atemplate, QRS width, beat-to-beat stability, or other factors. Intervalor rate variability may be monitored, and/or sudden onset oracceleration may be analyzed. In some examples, analysis 14 maydetermine that a detected event occurred due to noise or overdetectionand, if so, the system may return to detection loop 12.

The method continues to block 16 and determines an initialidentification of a treatable condition should be made. Block 16 mayinclude further analysis of an overall rhythm or cardiac state. In oneexample, block 16 reviews the adjudication by analysis block 14 of anumber of detected events, for example, 8, 12, 16, 24, 32, 40 or more orsome other number of events. An X/Y filter would then be generated bylooking, for example, for 18/24 detected events to demonstrate aparticular characteristic such as high rate, a particular morphology, amismatch to a morphology template, large width, high variability, orsome mix thereof. In another example, an asynchronous block of time maybe analyzed at block 16.

If the outcome of Treatable Condition block 16 is No, the method returnsto the detection loop 12. If the outcome of the Treatable Conditionblock 16 is Yes, the method continues to the Initial Treatable ConditionFound state.

The Initial Treatable Condition Found State allows the device to preparefor therapy delivery. At this point an episode can be declared by theICSD. The term “episode” indicates a particular state of the ICSD thatcan be called in response to identification of a condition that can beclosely analyzed and, if desired, recorded for later download by aphysician, such as a potential/likely arrhythmia. During an episode, theICSD may prepare to deliver therapy, deliver therapy, perform additionaldata analysis, record and store sensed/detected signals for laterretrieval, and/or generate a warning (vibration or emitted sound, forexample) to indicate therapy may be imminent. In addition, an Episodemay cause the ICSD to attempt communications with an external devicesuch as a bedside monitor or hospital information network or any otherexternal system adapted to receive ICSD communication such as a cellulartelephone network, WI-FI, Bluetooth, etc.

Episodes typically end when the identified condition ends, for example,following spontaneous reversion to a benign rhythm or return to benignrhythm after therapy delivery. If therapy is delivered and fails, somesystems will repeat therapy delivery until the arrhythmia is terminatedor a maximum number of therapy attempts have been performed. A systemmay also progress through a number of different therapies, for exampleapplying ATP in one or more formats and then moving on to cardioversionor defibrillation if the arrhythmia is not terminated. Where multipletherapies (repeating the same therapy or applying different therapies)are delivered, the methods shown herein may be repeated for each therapydelivery sequence. Alternatively, in another embodiment, theconfirmation processes may be performed for only the first therapydelivery of an episode.

In the Initial Treatable Condition Found state, the method again uses adetection loop 18 to determine whether an event has been detected.Detection loop 18 and detection loop 12 may apply similar or differentcriteria to identify detected events, depending upon the embodimentand/or physician preferences. Once a detected event occurs, the methodgoes to block 20, which performs analysis and initiates a count ofindications of benign cardiac conditions.

The analysis in block 20 can be performed, in an illustrative example,by counting how many indications of benign cardiac conditions appearduring the episode. The analysis from block 20 can be used to delaytherapy delivery if numerous indications of benign cardiac conditionhave appeared and, conversely, if no such benign indications haveappeared, the analysis in block 20 will allow therapy to be deliveredonce ready. The analysis in block 20 can observe various types ofmarkers of benign cardiac activity including, in some examples, slowrate detections.

In addition, the analysis in block 20 may also track the time series ofevents that indicate either benign or treatable conditions. For example,confirmation criteria may be changed in response to detected eventssuggesting a benign condition. If subsequent events suggest a treatablecondition has returned, the confirmation threshold may be returned toits original state.

Following analysis at 20, the method determines whether the episode hasended, as shown at 22. In an illustrative example, the episode isconsidered ended if a calculated rate is below a set threshold for apredetermined number of events, calculations or period of time. Forexample, if the calculated rate is below 140 beats per minute (BPM) for24 consecutive detections, the episode is considered terminated. Inanother example, if the average calculated rate for a set of 25calculations is less than 120 BPM, the episode is considered terminated.In yet another example, if the rate is less than 100 BPM in 20/25previous calculations, the episode is considered terminated. Othercriteria may be used in other embodiments in order to determine that theepisode has ended. If the episode has ended at 22, the method proceedsto a cleanup block 26.

The cleanup block 26 may include steps for discharging thedefibrillation capacitor (if it was charged in the Initial TreatableCondition Found State) and storing data for the terminated episode.Cleanup 26 may include other steps including, for example, annunciationor attempts to initiate communication with a home monitoring system.Following cleanup 26, the method returns to the Not Concerned state asindicated at 28. If desired, cleanup 26 may include changes to theanalysis of the Treatable Condition block 16 in response to anonsustained episode. Some examples are disclosed in US PatentApplication Publication Numbers 2009-0131998 and 2006-0167503, bothtitled METHOD FOR ADAPTING CHARGE INITIATION FOR AN IMPLANTABLECARDIOVERTER-DEFIBRILLATOR, as well as US Patent Application PublicationNumber 2006-0167504, titled DEVICES FOR ADAPTING CHARGE INITIATION FORAN IMPLANTABLE CARDIOVERTER-DEFIBRILLATOR, the disclosures of which areincorporated herein by reference. In one illustrative example, apersistence factor used in the Treatable Condition block 16 is modifiedin response to an episode ending without therapy delivery. For example,an X/Y counter may be used to determine how many Treatable detections(X) out of a set of Y detections have been encountered. When the X/Ycounter reaches a predetermined threshold and stays at or above thethreshold for a persistence number (N) of consecutive events, the chargebegin block 16 is satisfied. Following termination of an episode withouttherapy delivery, the persistence number (N) may be increased for use ina future episode. Other factors, such as the size of the X/Y set and/orthe X/Y ratio may be changed, instead.

If the episode has not ended at block 22, the method determines whetherthe system is ready for therapy delivery, as shown at 24. If so, themethod continues to the Confirmation state. If not, the method returnsto the detection loop 18. The check at 24 may include, for example,determining how many detected events have occurred while in the InitialTreatable Condition Found state, how long the Initial TreatableCondition Found state has been ongoing, and/or whether any therapycapacitor charging is complete.

In the Confirmation state, analysis again starts in a Detection Loop 32.Following a detection at 32, the method analyzes the detected event andsensed signal to count indications of benign rhythm (denoted in theexample as indications of Normal Sinus Rhythm, NSR), as shown at 34. Themethod next determines whether the episode has ended, as shown at 36.Finally, an Interval condition is applied as shown at 38. The Intervalcondition determines whether the intervals between detected events areshorter than a short interval threshold for at least a threshold numberof intervals. In an illustrative example, the short interval thresholdis an arrhythmia rate threshold that can be set via programmercommunications to a value corresponding to a rate of 140 to 270 BPM. Thethreshold number of intervals is determined by adding up the number ofindications of NSR that were counted in blocks 20 and 34, plus a defaultminimum number. If benign conditions are detected, therapy delivery canbe delayed until additional indications of treatable conditions areidentified. If desired, the sum of indications of NSR from blocks 20 and34 may be discounted in response to indications of VF or VT whenapplying the condition at 38.

If the Interval condition fails at 38, the method returns to thedetection loop at 32, staying in the Confirmation state. If the Intervalcondition 38 is met, therapy is delivered, as indicated at 40. Iftherapy is delivered at 40, the method will continue within the sameepisode, using a post-therapy state (not shown) including detection andanalysis to determine whether to start additional therapy cycles, aswell as analysis to determine whether therapy has been successful andthe episode has ended.

FIG. 2 shows an illustrative implantable medical device and implantlocation. More particularly, an illustrative subcutaneous-only ICSDsystem is shown in FIG. 2. The system is shown relative to a heart 50,and includes a canister 52 coupled to a lead 56. The canister 52preferably houses operational circuitry for performing analysis ofcardiac activity and for providing a therapy output. The operationalcircuitry may include batteries, input/output circuitry, powercapacitor(s), high voltage charging circuit(s), logic circuits,controller(s), memory, communication components, etc., as known in theart.

Electrodes are disposed at locations throughout the system including,for example, an electrode 54 on the canister 52, and electrodes 58, 60,62 on lead 56. The electrodes 54, 58, 60, 62 may take any suitable formand can be made of any suitable material. For example, the canisterelectrode 54 may be an isolated button electrode or it may be a regionor surface of the canister 52, and the electrodes 58, 60, 62 on lead 56may be coil electrodes, ring electrodes, or other structures known inthe art. More or fewer electrodes may be provided on the canister 52 andor the lead 56.

The electrodes 54, 58, 60, 62 define a plurality of sensing vectors suchas V1, V2, V3 and V4. If desired, one or more vectors V1, V2, V3, and V4may be chosen as a default sensing vector, for example, as discussed inUS Patent Application Publication Number 2007-0276445 titled SYSTEMS ANDMETHODS FOR SENSING VECTOR SELECTION IN AN IMPLANTABLE MEDICAL DEVICEand/or U.S. Pat. No. 7,392,085 titled MULTIPLE ELECTRODE VECTORS FORIMPLANTABLE CARDIAC TREATMENT DEVICES, the disclosures of which areincorporated herein by reference. Another embodiment considers posturein vector analysis, for example, as discussed in US Patent ApplicationPublication Number 2008-0188901 titled SENSING VECTOR SELECTION IN ACARDIAC STIMULUS DEVICE WITH POSTURAL ASSESSMENT, the disclosure ofwhich is incorporated herein by reference. Multiple sensing vectors maybe analyzed, sequentially or in combination, as desired.

Therapy may be applied using any chosen pair of electrodes. Anillustrative example uses the can electrode 54 and the coil electrode 62to apply therapy. Other electrode combinations may be used. Therapy mayinclude mono-, bi- or other multi-phasic defibrillation and/or variouspacing operations.

FIG. 2 omits several anatomical landmarks. The illustrative system shownmay be implanted beneath the skin outside of the ribcage of theimplantee. The location illustratively shown would place the canister 52at approximately the left axilla of the implantee, level with thecardiac apex, with the lead 56 extending medially toward the xiphoid andthen toward the head of the implantee along the left side of thesternum. One illustrative example uses a method/system as shown incommonly assigned US Patent Application Publication Number 2006-0122676entitled APPARATUS AND METHOD FOR SUBCUTANEOUS ELECTRODE INSERTION, thedisclosure of which is incorporated herein by reference. Otherillustrative subcutaneous systems and locations are shown in commonlyassigned U.S. Pat. Nos. 6,647,292, 6,721,597 and 7,149,575, thedisclosures of which are incorporated herein by reference.

The present invention may also be embodied in systems having variousimplant configurations including but not limited to othersubcutaneous-only, vascular-only, epicardial, and/or transvenousimplantation configurations and locations. The canister 52 may be placedin anterior, lateral, and/or posterior positions including, withoutlimitation, axillary, sub-clavicular, pectoral, and sub-pectoralpositions, as well as placements on either the left or right side of theimplantee's torso and/or in the abdomen. The canister may be a singleunit or it may comprise a number of connected or tethered enclosures invarious designs known throughout the art. Entirely intravascularimplantation of an ICSD system has also been proposed. The lead 56 maybe placed in any of a number of suitable configurations includinganterior-posterior combinations, anterior-only combinations, transvenousplacement, or other vascular placements. Multiple leads 56 may be usedas well. A unitary system that omits lead 56 and includes all electrodeson the canister 52 may also be used.

The present invention is not intended to be limited to any particularhardware, implant location or configuration. Instead, it is intended foruse in any implantable cardiac system. In addition to therapy deliverysystems, some embodiments may include monitoring systems. For example,monitoring functions such as annunciation or data storage may bemanipulated, rather than controlling therapy delivery. A monitoringsystem could also be used to demonstrate the suitability of analyticalmethods for a particular patient.

Some examples can associate with an external programmer 64 configured tocommunicate with an implanted device for various purposes, including,for example and without limitation, one or more of the following: devicetesting; upload new/revised software/firmware; modify sensing, detectionor therapy settings; determine the status or history of deviceoperation, battery life, or lead integrity; and/or download datarelating to the device or implantee's condition, prior data capture, ortreatment. Any suitable communication method may be used, such asvarious protocols and hardware widely known in the art including, forexample, MICS, inductive telemetry, RF telemetry, Bluetooth, etc.

FIG. 3 shows, in block form, an illustrative method of operating an ICSDfor purposes of delivering relatively high-energy therapy such asdefibrillation or cardioversion. In the illustrative method, a newdetection 80 is the trigger for a series of analysis steps. A newdetection 80 may be identified, for example, by capturing a cardiacsignal from implanted electrodes associated with the system andcomparing the sensed signal to a detection threshold. A detectionthreshold may be fixed or it may be a time-varying threshold, forexample as shown in US Patent Application Publication Number2009-0228057, titled ACCURATE CARDIAC EVENT DETECTION IN AN IMPLANTABLECARDIAC STIMULUS DEVICE, the disclosure of which is incorporated hereinby reference. Other detection methods may be used instead.

In the illustrative example, following a new detection 80, NoiseDetection is performed as shown at 82. Noise Detection 82 may beperformed to identify detections that appear to be caused by noise orsubstantially masked with noise. If a detection appears to be caused bynoise or substantially masked with noise, the method can return to block80 and await the next detection.

Otherwise, Overdetection Identification 84 is performed. OverdetectionIdentification may take any of several forms. Overdetectionidentification 84 is included to identify (and correct, if suitable)instances where more than one detection occurs within a single cardiaccycle such as double or triple detection.

Rhythm Analysis 86 follows, in which the patient's overall cardiacrhythm is analyzed to determine whether a treatable arrhythmias. Ratewill often be a factor in Rhythm Analysis 86. In some examples, rate maybe analyzed before blocks 84 or 86 and, if the calculated rate is low,the method may return to block 80. Morphology, width, and other factorsmay be part of Rhythm Analysis 86.

If the Rhythm Analysis 86 finds that the patient's cardiac rhythm isbenign and does not require treatment, the method returns to block 80and awaits the next detection. If Rhythm Analysis 86 identifies atreatable arrhythmia, Therapy block 88 is called. Therapy block 88 mayinclude charging 90 for the cardioversion/defibrillation capacitor inthe ICSD. If charging 90 is not complete, the method returns to block 80for a next iteration.

Once charging 90 is complete, Therapy block 88 confirms ongoing need fortherapy and, if therapy is confirmed, therapy is delivered at 92. Ifconfirmation fails, the method again returns to block 80 for a nextiteration. In another example, ATP can be provided, in which case thecharging block 90 can be omitted or replaced with another analysis blockthat observes whether the device is ready to deliver therapy. Whetherthe device is ready to deliver therapy may be determined by, forexample, meeting confirmation criteria, observing that the treatablecondition has not terminated in a predetermined period of time, ordetermining that an arrhythmia is progressing to a worse state than wasinitially detected.

The Noise Detection referenced at block 82 of FIG. 3 may includewaveform appraisal as described in commonly assigned U.S. Pat. No.7,248,921, titled METHOD AND DEVICES FOR PERFORMING CARDIAC WAVEFORMAPPRAISAL, and/or U.S. Provisional Patent Application No. 61/255,253,titled ADAPTIVE WAVEFORM APPRAISAL IN AN IMPLANTABLE CARDIAC SYSTEM, thedisclosures of which are incorporated herein by reference. Overdetectionidentification 84 may be performed, for example, using methods such asshown in US Patent Application Publication Number 2009-0259271, titledMETHODS AND DEVICES FOR ACCURATELY CLASSIFYING CARDIAC ACTIVITY, and/orUS Patent Application Publication Number 2010-0004713, titled METHODSAND DEVICES FOR ACCURATELY CLASSIFYING CARDIAC ACTIVITY, the disclosuresof which are incorporated herein by reference. Rhythm analysis 86 mayinclude analysis as disclosed in the 2009-0228057, 2009-0259271 or2010-0004713 Application Publications, the U.S. Pat. No. 7,248,921and/or commonly assigned U.S. Pat. No. 7,330,757 or U.S. Pat. No.6,754,528, the disclosures of which are incorporated herein byreference. Other methods may be used instead of or in addition to any ofthe examples found in these patents and patent applications.

FIG. 4 is a process flow diagram for an illustrative method for adaptingthe conditions for therapy delivery to a detected rhythm. The managementof therapy delivery in FIG. 4 accounts for indications of benign cardiacactivity before delivery of therapy. As a result, therapy delivery maybe avoided in response to relatively late reversion to a benign cardiacrhythm. Before explaining the solution posed by the method illustratedin FIG. 4, FIG. 5 shows an example of therapy delivery that could beavoided by counting indications of benign cardiac activity.

Turning to FIG. 5, a sensed signal is shown at 150. The signal 150 ischaracterized at 152. The illustrative system relies on an average ofcalculated intervals between detected events to generate an Event Rate,which is characterized at 154. In the example an X/Y counter is used todetermine how many treatable events (X) out of a set of detected events(Y) occur. The contents of the X/Y counter are characterized at 156.Starting from the left, the patient is in a normal rhythm. A high ratearrhythmia begins that, as indicated at 154, drives the Event Rate high(relative to a programmed rate threshold), and, as indicated at 156, theX/Y counter begins to fill with treatable events.

When the X/Y counter meets certain analysis threshold(s), chargingbegins, as shown at 158. Before charging is completed 160, thearrhythmia spontaneously reverts and slows down to a normal sinusrhythm, as shown at 162. This drops the Event Rate to a low zone.Therapy delivery after the high rate arrhythmia terminates is likely notneeded. However, during the normal sinus rhythm 162, the patientencounters a short series of detected events caused by ventricularextrasystoles (VES), shown at 164. A premature ventricular contractionis one type of VES.

In the illustrative example, the confirmation criteria include thefollowing: if three consecutive intervals are shorter than apredetermined threshold, therapy is delivered. Here, the VES cause aseries of very short intervals between detected events. Once three shortintervals occur, therapy is delivered, as shown at 168. The therapy inthese circumstances is likely clinically unnecessary and may be painfuland/or startling to the patient.

The method of FIG. 4 may avoid therapy as shown in FIG. 5. The methodillustrated by FIG. 4 may be used at blocks 90 and/or 92 in FIG. 3, forexample. The method begins at the start block 100; the conditionsprecedent to start block 100 are the occurrence of a newly detectedevent, and, optionally, initial identification of a treatable cardiaccondition.

From start block 100, the method determines whether a detected eventunder analysis has been identified as non-Treatable, as indicated at102. If so, then a Therapy Persist Threshold, TPTh, is incremented.Optionally, a maximum value for TPTh may be set, for example, a maximummay be in the range of about 8-64, with larger or smaller valuespossible depending upon how the variable is used. In one example, themaximum value for TPTh is 24.

Returning to block 102, if the event under analysis has not been markedas non-Treatable, the method next determines if the event under analysishas been marked as Treatable, as shown at 106. If so, then TPTh isdecremented, as shown at 108. If desired, TPTh may have a lower limit,as shown in FIG. 4. In an illustrative example, TPTh has a lower limitof three.

In the example of FIG. 4, if the event under consideration is marked asNoise, for example based on Noise Detection (FIG. 3 at 82), it may passthrough without incrementing 104 or decrementing 108. Following theanalysis of Treatable/nonTreatable 102, 106 and increment/decrementsteps 104, 108, if needed, the method continues to block 110.

At block 110, the method determines whether the ICSD is ready fortherapy. Block 110 may, for example, determine that a minimum period oftime has passed since the initial identification of a treatablecondition, and/or it may include determining that capacitor charging iscomplete, indicating that the ICSD defibrillation capacitor(s) has beencharged to a desired voltage/energy level. If the ICSD is not ready fortherapy delivery, the method stops as indicated, and waits for anothernew detection.

If the conditions of block 110 are satisfied, the method next determineswhether it is following a first iteration, as shown at 112, afterdeclaring ready for therapy at block 110. If so, a variable, TreatmentPersist Count (TPCount), is initialized to the number of Treatableintervals that have occurred in the last three detected intervals, asindicated at 114. If not in the first iteration, the method continues byobserving whether the most recent detected interval is shorter than athreshold for short intervals, as shown at 116. If so, TPCount isincremented by one, as shown at 118 and, if not, TPCount is reset tozero, as shown at 120. In this example, when a string of short intervalsis identified, TPCount will increase with each new short interval asshown at 118, while a long interval will cause TPCount to reset to zeroas shown at 120. If TPCount is reset at 120, then the method stops, asshown, and waits for a next iteration caused by a next detected event.

In the example of FIG. 4, a “short” interval is an interval that isshorter than a defined threshold that is indicative of a likelytreatable arrhythmia. In some examples, the defined threshold for shortintervals will be set based on a ventricular tachycardia (VT) rate, forexample, if a physician determines that rates above 200 BPM would betreatable tachycardia for a particular patient, the defined VT thresholdwould be 300 milliseconds. A shorter or longer interval may be used,depending on patient characteristics. If desired, a fibrillation-basedthreshold may be used. For example, ventricular fibrillation (VF) may bedeclared for rates above a threshold such as 240 BPM, and a shortinterval may be an interval shorter than 250 milliseconds. The systemmay be customized for a given patient by adjusting these thresholds.Other “short” intervals unrelated to VT or VF thresholds may be usedinstead, and the calculation of TPCount does not need to be tied todefined therapy or rhythm classification thresholds.

Following Initialization at 114 or an increment at 118, the methodcontinues to 122, where TPCount is compared to TPTh. If TPCount isgreater than or equal to TPTh, the method then performs final therapypreparations (if needed) and delivers therapy to the patient as shown at124. Alternatively, if TPCount is not greater than or equal to TPTh, themethod stops and awaits the next detection.

As can be seen, the method in FIG. 4 will count long intervals and usethese as indications of non-treatable cardiac rhythm to extend TPTh. Asa result, if indications of benign cardiac rhythm are identified, themethod causes the system to wait to deliver therapy for some number ofadditional treatable detections. This may avoid therapy delivery if apatient experiences spontaneous reversion to benign cardiac rhythmand/or receives successful external defibrillation therapy. Meanwhile,if the system detects few or no nontreatable or long interval events,TPTh remains small, and the patient will receive therapy shortly afterthe ICSD meets readiness conditions at 110.

Any number of features may be used to identify benign cardiacconditions. In one example, comparison to a normal beat template may beused to identify indications of benign cardiac rhythm. A normal beattemplate may be a template captured at a resting heart rate asrepresentative of Normal Sinus Rhythm, and/or a normal beat template mayrepresent a heart beat at elevated rate conditions such as exerciseinduced tachycardia. High correlation to a normal beat template may bean indication of benign cardiac conditions, even with concurrent highdetected cardiac rate.

The confirmation concept can be used to confirm therapy delivery of anysort including ATP, cardioversion or defibrillation. Furthermore, theseillustrative examples may be used in treatment of ventricular or atrialconditions. For example, a confirmation step may be applied beforedelivering therapy for polymorphic ventricular tachycardia, ventricularfibrillation, atrial fibrillation or flutter, for example. Dependingupon patient needs, other conditions may also be treated.

The present invention is not limited to any particular method ofidentifying indicators of treatable or non-treatable cardiac conditions.Several examples refer to marking an event as treatable or nottreatable, where “event” refers to individual detected events and notthe overall episode (for example, an episode may include a period ofventricular fibrillation including a number of treatable detectedevents). In some examples, rather than marking “events” as treatable,the intervals associated with detected events can be marked as treatableor not treatable. For example, rate-only analysis may mark intervalsbetween detections as treatable or not treatable; a morphology-onlyanalysis may mark the individual detections as treatable or nottreatable. In a hybrid of these analyses, an interval between detectedevents is associated with either the detected event that precedes theinterval or the detected event that ends the interval, and themorphology of the event and the duration of the interval may each factorinto marking the associated pair of detected event and interval astreatable or not treatable. In yet another example, the morphology ofthe signal during an interval between detected events can be analyzed toidentify treatable conditions as well.

In some examples, the confirmatory criteria may add additional datainputs to the analysis by, for example, making reference to acceleratordata (indicating activity or posture), blood pressure sensors, bloodcomposition sensors or other data. Such additional data inputs can beused in combination with the above described confirmatory criteria.

FIGS. 6A-6B show analysis in a format similar to FIG. 5, but using themethods shown in FIG. 4. Referring to FIG. 6A, the top of the drawingshows sensed cardiac signals at 170. The cardiac signals arecharacterized at 172. The ICSD analysis of the signal is indicated inpart by the Event Rate, characterized at 174, and the status of the X/Ycounter shown at 176.

The status of the system's Charger, which is used to charge a therapycapacitor, is shown at 178. The example of FIGS. 6A-6B illustrates asystem preparing for a defibrillation or cardioversion therapy; in otherexamples, rather than charger status, a timer status could be shown ascounting how long a treatable condition persists before deliveringtherapy, for example, if the therapy does not require charging. Forexample, before delivering ATP the system may wait for a predeterminedperiod of time.

Starting from the left, the signal 170 is initially shown as normal at172, with low rate 174 and the X/Y counter 176 is low/off. Given thenormal rhythm, charging has not started. Following the signal across tothe right, the patient then develops a tachycardia, as shown at 170, andcharacterized at 172, leading to a high rate calculation 174. The X/Ycounter 176 begins to count indications of treatable condition based onthe high Event Rate. The charger remains off, pending satisfaction ofrhythm criteria.

In the example, the X/Y counter is used to apply rhythm criteria bycounting the number of detections that indicate a treatable arrhythmia.In this example, the high rate calculation 174 indicates a treatablearrhythmia, and the X/Y counter begins incrementing with each detectedevent that meets the high rate criteria, until a threshold is met. Forexample, an X/Y counter threshold may be of the nature of 5/8, 9/12,12/16, 14/18, 18/24, etc., usually requiring a supermajority (such as75% or some other proportion) of a set of events to indicate a treatablearrhythmia. A persistence rule may also apply, in which the X/Y counteris called upon to meet a threshold for a specified number of consecutivedetections or, alternatively, a period of time. For example, persistencemay wait until the X/Y counter meets its threshold for at least 2consecutive measurements, one full second, or some other suitablemeasure. The persistence rule may increase in duration if priornonsustained treatable conditions have occurred.

Once the X/Y counter criteria is met (including any persistence factor),the Charger begins charging, as shown at 178. Once the charger beginscharging, the system begins calculating a Therapy Persist Threshold(TPTh) and a Therapy Persist Count (TPC) as shown at 180. TPC representsthe number of consecutive short intervals currently detected. In thisexample, TPC counts up as new short intervals are detected, and is resetto zero when a long interval occurs. TPTh records the threshold numberthat TPC must meet or exceed in order to satisfy Therapy Persistcriteria. TPTh varies in response to detected indications of benign andtreatable conditions.

In the example, the signal returns to a slow rate (NSR) shortly aftercharging begins, causing long intervals and lowering the ratecalculation 174. As shown at 180, the TPTh monitor begins to count upthe number of long intervals when the NSR starts to occur after chargebegin. These long intervals are treated in this example as indicationsof benign cardiac rhythm. TPC is set to zero due to the long intervals.Charging completes as shown at 178 while the NSR is still continuing andthe TPTh is still counting up, so the Charger enters a Wait state. TheCharger will remain in a Wait state until a Confirmation rule(TPC.gtoreq.TPTh) is satisfied and therapy is delivered, or until theepisode ends based on the system's determination that an arrhythmia isno longer occurring.

In an illustrative example, the episode can be terminated if systemdetects enough slow rhythm beats to determine that the treatablearrhythmia has reverted to a benign rhythm. For example, a fixed numberof consecutive long intervals may be required, the X/Y counter may countdown to zero, or rate calculated using average intervals may show a lowrate for a predetermined period of time or number of consecutivecalculations. In one example, if twenty-four low rate calculations 174appear in a row, a declared episode is considered terminated. In anotherexample, if the X/Y counter reaches three or lower (or zero, in yetanother example), the episode is considered terminated. In anotherexample, if a low rate is detected for a period of time in the range of10-20 seconds, the episode can be considered terminated.

Returning to FIG. 6A, a short burst of VES occurs and causes the rate174 to momentarily go High which, in turn, begins allowing the method tostart counting down TPTh. TPC also starts counting up, but because TPThcounted up during the NSR after charging began, TPC stays less thanTPTh. The outcome is that therapy is not delivered in response to theseries of VES.

Later, a short burst of VT occurs, again driving the rate 174 high andcausing TPC to begin counting up while TPTh counts down. Once again, theincrementing of TPTh during slow rate beats appropriately preventstherapy delivery. The mathematics of this analysis are furtherhighlighted in FIG. 6B.

FIG. 6B provides further details to the analysis of FIG. 6A, focusing onwhat happens once Charging begins. A portion of the signal from FIG. 6Ais shown at 182, with the end of the high rate rhythm that causedcharging to begin shown at 184. The values for variable TPTh and TPC areshown below the signal 182. In particular, when charging begins, TPTh isinitialized to 3. As shown, during the continuing high rate conditionsdemonstrated at 184, TPTh stays constant at 3. Once a slow ratecondition begins, TPTh starts to count up.

In some examples, TPC is not calculated during charging. In otherexamples, TPC may be calculated during charging to allow confirmation oftherapy delivery as soon as the charging process is completed, withouthaving to wait for TPC to increment up to the lowest value of TPTh. Theillustrative example shown in FIG. 6B does not calculate TPC untilcharging is complete, as indicated by the asterisks.

Once charging ends, in this example, TPC is initialized to zero. In analternative example, TPC may be initialized based on a series ofpreviously detected events. The illustrative example is iterative, andTCP and TPTh are recalculated by incrementing or decrementing as newdetections occur. As shown at 186, TPTh counts up by several incrementsbefore a first VES detection appears. As a result, following the firstshort interval caused by the VES, TPTh=11 and TPC=1. Each of thesevalues is updated with new detections and, at the end of several shortintervals caused by the VES, TPTh=8 and TPC=4.

In this example, a 4-interval average is used to estimate rate for TPThcalculations, while the instantaneous interval is used for TPC. SinceTPTh is generated from a 4-Interval average in the detailed exampleshown, it does not immediately begin counting down once the shortintervals from the VES occur, as these are averaged in with priorintervals. The use of a 4-interval average is merely illustrative, andother calculations may be performed to estimate rate instead. TPC andTPTh can be based on the same calculation (as shown) or differentcalculations in various embodiments. The next interval is long, causingTPC to be reset to zero, while TPTh is decremented to seven since it isbased on a 4-Interval average.

Following the VES, TPTh again begins to count up. When the first shortinterval of the VT is detected, TPTh=15, as shown at 188. As shown at190, when the VT terminates, TPTh=10 and TPC=6. The next calculation, asshown at 192, shows reset of TPC=0, while TPTh=9, as it continues todecrement for one more iteration since it relies on an averagedinterval. As can be seen, due to TPC and TPTh, therapy is appropriatelywithheld even in the presence of a nonsustained VT and several shortintervals at the VES triplet.

In the example shown, TPC counts up while TPTh counts down, effectivelyhalving the delay. Some examples avoid this halving effect, for example,TPTh may increment twice for each long interval, and decrement once foreach short interval. In addition, to avoid putting off therapy for anundesirably long period of time, TPTh may be capped, for example, with amaximum value in the range of 8-64, or less or more. In one example,TPTh is capped at 24. Other maximum values for TPTh can be used.

FIG. 6C shows an example that avoids the halving effect caused when TPThcounts down while TPC counts up (shown by FIG. 6B, in particular). Acardiac signal is shown at 200, with a treatable tachycardia representedat 202. The tachycardia 202 causes charging to begin. A new variable,TPTh(x), is used to retain the value of TPTh at the start of a series ofshort intervals. In the example, during charging the signal 200 revertsto a normal rhythm with long intervals, causing TPTh to count upward.Next, a ventricular fibrillation (VF) starts, as indicated. At the timethe VF starts impacting rate, TPTh=15, as shown at 204. When TPC=1, asshown at 206, TPTh(x) is “Set” to equal TPTh, as shown at 208. TPTh(x)retains the “Set” value until the series of short intervals is brokenand restarts—that is, until TPC is reset to zero and begins counting upagain.

In the example, a large amplitude detection interrupts the detection ofVF, leading to undercounting or dropout. As shown at 210, when the largeamplitude detection occurs, TPC is still less than TPTh(x). In theexample, therapy is inhibited until TPC=TPTh(x), so no therapy is yetdelivered. Undercounting following the large amplitude event causes areset of TPC, and when VF resumes again after the dropout caused by theVES, TPTh(x) is set to the then-current value of TPTh. Therefore, asshown at 212, TPTh(x) is set to equal TPTh, which was at 8 at the time,for use going forward. Because the VF continues, the system proceeds todeliver therapy once TPC=TPTh(x). If the VF were to terminate and returnto normal rhythm, for example after the large amplitude event, notherapy would have been delivered in the example.

In another example using ATP and/or where the capacitor charging is notneeded or has short duration, a minimum time period may be defined toforce a delay period into the system. Thus, rather than showing “ChargeBegin” and “Charge End” 220, the method may instead use Timer Start andTimer End 222. The delay may be in the range of 3-10 seconds, or more orless.

FIGS. 7A-7B show a process flow for another illustrative method formanaging therapy delivery. At a summary level, the example of FIGS.7A-7B relies on factors related to an X/Y counter to determine whetherto make an initial identification of a treatable condition and,following initial identification of a treatable condition, the X/Ycounter is cleared and new data refills the X/Y counter, ensuring thatthe arrhythmic condition is continuing before therapy is delivered FIGS.7A-7B show various conditional steps using “Charge” states; in anotherexample, a timer may be referenced instead, as noted at 290. In yetanother example, rather than a timer, a selected number of detectedevents must occur while the ICSD waits to declare itself ready fortherapy.

The method begins in FIG. 7A at an event detection block 250, whichresponds to events detected in a sensed cardiac signal. When an event isdetected at 250, the method proceeds to beat analysis at 252. Beatanalysis 252 may include, for example, any suitable noise-identificationanalysis and/or analysis to identify double detection or other detectionanomalies.

If beat analysis 252 fails (indicating that the detected event is notlikely a cardiac event of a desired type), the method returns to 250. Ifbeat analysis passes, the method goes to block 254, where the X/Ycounter is adjusted according to the beat analysis 252. This mayintegrate features associated with rate and/or morphology analysis todetermine whether a detected event indicates a benign cardiac state or atreatable condition.

Next, the method determines whether charging has already begun, asindicated at 256. If so, the method continues to block 258, whichdetermines whether charging is complete. If charging is complete, themethod goes to FIG. 7B, via “B”. In the alternative 290, the methodwould determine whether a therapy delay timer has started and, if so,whether the timer has expired at blocks 256 and 258, respectively.

If charging has started but is not complete, the method next determineswhether the arrhythmia has reverted to benign rhythm and the episode canend, as shown at 260. If the arrhythmia has reverted, various steps canbe taken to end the episode, for example, data stored in temporarymemory may be written to a memory storage location for later retrieval,and the defibrillation capacitor (if it was charging) may be discharged.Any suitable adjustments that are desired following a non-sustainedevent may be made as well, for example, by adjusting persistencecriteria. After block 262, or if the arrhythmia has not reverted at 260,the method goes to “A”, which leads back to beat detection 250.

Returning to block 256, if charging has not already begun (or thetherapy timer has not started), the method continues to block 264, wherethe contents and history of the X/Y counter are analyzed to determinewhether charging should begin (or the therapy timer should start). Ifnot, the method again goes to “A”, which leads back to beat detection250.

If block 264 finds that charging should begin, then charging isinitiated, as shown at 266, which may include sending or recording acharge begin marker (used at 256), or declaring that an episode hasbegun. In the alternative 290, a therapy timer can be initialized atblock 264.

In the illustrative example, once charging is initiated at block 266,the X/Y counter is cleared, as shown at 268. In some examples, thismeans emptying the X/Y counter completely; while in other embodimentsthe X/Y counter may be set to some predetermined initial condition forthe post-charge-begin analysis. For example, the X/Y counter may be a 20event counter (that is, Y=20) and “clearing” at block 268 could set X tozero, ten, or some other predetermined value. In the alternative 290,if/when the therapy timer is initialized, the X/Y counter would becleared. Following block 268, the method goes to “A” and returns to beatdetection 250.

Turning to FIG. 7B, after charging is complete (at 258), (and/or, usingthe alternative 290, if the therapy timer has expired) the illustrativemethod next determines whether a first therapy in the present episodehas been delivered already, as shown at 270. If not, then the methoddetermines whether the X/Y criteria has been met after the charge beginand data clearing steps at 266 and 268 (FIG. 7A). In another alternativeexample, a therapy timer may be omitted after the first therapy isdelivered in an episode, or a therapy timer may be selectively appliedwhen only certain therapies are being considered for example in a systemconfigured to apply multiple different therapies in a predeterminedorder or in response to particular conditions.

The thresholds applied before and after charge initiation in the methodof FIGS. 7A-7B may be different from one another in some embodiments.The following combinations are illustrative of X/Y thresholds that canbe used in the illustrative example:

Initial ID X/Y=18/24, with variable Persistence; Confirmation CriteriaX/Y=12/16, No Persistence.

Initial ID X/Y=12/16, variable persistence; Confirmation CriteriaX/Y=10/16, No Persistence.

Initial ID X/Y=18/24 (up to 30/40), variable Persistence; ConfirmationCriteria X/Y=12/18, no persistence.

In these examples, Persistence refers to requiring the X/Y ratio to beshown for a series of M consecutive calculations, Static Persistencecalls for M to be fixed, and Variable Persistence indicates that M canbe increased/extended if a non-sustained tachycardia is identified.Other values than those shown may be used. Alternatively, the samecriteria is reapplied after charge begin. These examples may becharacterized as offering an initial identification criteria and aconfirmation criteria.

If the X/Y rule(s) are met at 272, the method optionally applies a “lastthree interval” rule as shown at 274, in which the last three intervals(raw, noise-detection-passing, or even certified by overcountingdetection methods, depending on which of several embodiments is used)are analyzed to determine whether each is short. If the last threeinterval rule is not met, the method continues back to block 250 in FIG.7A via “C”. If the last three interval rule is met, the method deliverstherapy, as shown at 276.

The last three interval rule may be applied if the first therapy of theepisode has already been delivered, advancing from block 270 as shown.In another embodiment, the last three interval rule can be appliedbetween blocks 270 and 276, but is omitted between blocks 272 and 276.Rather than “last three,” a last-one, last-two or other number ofintervals may be checked, and, if desired, this embodiment may becombined with the embodiment of FIG. 4, in which the number of intervalschecked in the “last three” rule increases in response to indications ofbenign cardiac activity.

Following therapy delivery, any suitable post-therapy-deliveryoperations are performed as indicated at 278. Some examples ofpost-therapy-delivery operations 278 may include blanking, manipulatingswitches to eliminate afterpotentials, and, if desired, additionalfiltering of incoming signal to remove DC afterpotentials or to remedybaseline drift that may occur during and following therapy delivery.Post-therapy pacing can also be provided in appropriate circumstances,for example, as needed following defibrillation or cardioversiontherapy. The method returns to FIG. 7A via “C”.

Returning to block 272, if the X/Y criteria is not met when reapplied,the method next determines whether the episode has terminated, as shownat 280. In one example, the episode is considered terminated if thecalculated rate drops below a predetermined threshold. In anotherexample, the episode is not considered terminated until the calculatedrate drops below a predetermined threshold for a set number ofcalculations of the rate. For example, the treatable arrhythmia may beconsidered terminated when the detected event rate drops below 140 BPMfor 24 consecutive calculations. Other thresholds for determining thatthe episode has ended may be applied in other embodiments. For example,the rate threshold may be a programmable feature. If the episode hasterminated at 240, then End of Episode activities are performed as shownat 282. The End of Episode activities may be as described above. Themethod again returns to FIG. 7A from either of block 240 or 242 via “C”.

FIG. 8 illustrates analysis using the method of FIGS. 7A-7B. Arepresentation of a cardiac signal is shown at 302, and characterized at304. The patient can be seen to transition from a low rate or normalrhythm to VT, with spontaneous reversion to a low rate condition. Aseries of VES occurs as shown at 312. The calculated rate ischaracterized as shown at 306, and the status/contents of an X/Y counterare shown at 308. During the low rate rhythm, the rate 306 is low, andthe X/Y counter 308 is either Off or Low. When VT starts, the rate 306goes High, and the X/Y counter 308 begins counting up toward itsthreshold. Once the X/Y counter is met and an optional persistencefactor is satisfied, the ICSD has an initial identification of atreatable condition. The initial treatable condition is addressed in theillustrative system by beginning to charge a therapy capacitor, as shownat 310.

As also shown at 310, when charging begins, the X/Y counter is reset tozero. During charging, the X/Y counter begins to fill again withindications of Treatable and benign cardiac events. Once charging iscompleted, the system analyzes the contents of the X/Y counter todetermine whether an X/Y counter condition is met. As indicated, acounter requirement can be applied (confirmation criteria of 10/15, forexample) for confirmation than is used in the initial treatablecondition determination (X/Y criteria of 14/18, for example), andpersistence may be reduced in the illustrative example from three(charge criteria), to zero (therapy criteria).

If the Confirmation criteria is satisfied, therapy will be delivered.However, in the example shown, the VT that led to initiation of chargingspontaneously ends and the cardiac rhythm returns to a slow ratecondition. As a result, the X/Y criteria is not met when charging ends.A VES triplet is shown at 312. Because the VES may be of a differentamplitude or shape than other signals, overcounting can occur and/or maybe difficult to resolve, leading to a series of short intervals. Thismomentarily causes the calculated rate to increase and the X/Y countermay start to increment. However, the fast detections do not persist longenough in this example to meet the confirmation criteria being appliedvia the X/Y counter. As a result, no therapy is delivered, as noted at314.

As with other examples herein, rather than starting to charge a therapycapacitor in response to the initial treatable condition beingidentified, the ICSD may initialize a therapy timer or perform differentsteps to prepare for therapy delivery. In another example, because theX/Y counter is being refilled, no timer is needed and, instead, the X/Ycounter is cleared and if, as the X/Y counter fills, it again indicatesa treatable condition within a predetermined period of time (forexample, one minute), therapy is delivered.

It should be noted that the description “preparing to deliver therapy”in response to an initial identification of a treatable condition caninclude any number of different operations by the ICSD. For example,after an initial identification of a treatable condition, an ICSD canprepare to deliver therapy by one or more of: providing annunciation tothe patient; attempting external communication (communication with aprogrammer, bedside monitor, cellular network, etc.); charging acapacitor; recording signal data; measuring device parameters such asbattery capacity; identifying time windows for therapy delivery;performing sub-threshold testing of therapy delivery vectors; observingpatient impedance characteristics; measuring noncardiac signal noise; orperforming any other desired function. Depending upon the particularembodiment, these functions can continue until one or more of thefollowing occur: a therapy capacitor reaches a target voltage/energy; atimer expires; and/or a predetermined number of detected events occur,or any other suitable condition indicating that the ICSD is ready todeliver therapy occurs. For some systems and devices, simply waiting forexpiration of a timer is a preparation for therapy insofar as suchdelays can help ensure that non-sustained, transient conditions do notcause inappropriate therapy.

In another embodiment, an X/Y counter is used to analyze a window ofdata to identify a treatable condition. Upon initiation of charging, thevalues in the X/Y counter are stored (rather than cleared as in FIGS.7A-7B and 8), and analysis continues during charging, filling the X/Ycounter with new data. When charging is complete, the status of the X/Ycounter is compared to the status stored at the time charging wasinitiated. In the example, therapy would be delivered either immediatelyif the X/Y counter indicates a treatable condition that is at least ashazardous as that which existed at the time of charge initiation or oncethe X/Y counter reaches a status that is at least equivalent to thatwhich occurred at the time of charge initiation. Once again, persistencemay be omitted when performing the confirmation analysis of the X/Ycounter.

For example, if charging is called using a persistence=3 factor and anX/Y threshold of 18/24 (18 Treatable out of 24 events), charging maybegin at X/Y=21/24. At the end of the charging process, the system wouldthen observe whether X/Y is greater than or equal to 21/24 at least oncein order to trigger therapy delivery. If desired, a maximum X value(such as 22) may be set relative to Y, to ensure that application of apersistence rule does not lead to X/Y=24/24, which could be difficult toreach if the patient has any undersensing.

The above illustrative examples show several systems and methods inwhich cardiac activity is monitored after charging starts. In someexamples, if slow rate or normal cardiac events are detected aftercharging starts, additional therapy criteria are applied before atherapy will be delivered. In this fashion, a patient who displays someindications of benign cardiac activity during and following charging isrendered less likely to receive unnecessary therapy in response to aself-terminating arrhythmia.

In some examples, tiered analysis can be applied, having episodedeclaration criteria applied first, followed by therapy deliverycriteria, and methods as shown above may be used to affect the therapydelivery criteria subsequent to the episode declaration criteria, withor without a delay period intervening. Several of the above examplesmake note of a delay period during which capacitor charging occurs toprepare for therapy delivery. Not all systems or therapies will requiresuch a delay period; for example, if ATP is to be applied, therapydelivery may be available with no or very minimal delay period.

FIG. 9 is a block diagram for an illustrative method. The method can beperformed in an ICSD that senses cardiac signals from implantedelectrodes in order to determine whether a treatable cardiac arrhythmiais occurring. In response to an initial identification of treatablecardiac arrhythmia, the ICSD may prepare to deliver therapy, forexample, by charging high power capacitors to prepare to deliver therapyto a recipient of the ICSD.

As indicated at 400, the method begins with a determination that therapyis indicated. Determinations that therapy is indicated may be made byany method in this example. Once therapy is indicated, the methodoperates in a “Preparing for Therapy” block 402, until the ICSD is readyto deliver therapy, at which time the method operates in a“Confirmation” block 404. The methods are shown at 410-412/414-416 and420-422/424-426, and may be performed within larger processes, inparallel to other methods, or in addition to other processes including,for example, event detection.

The cardiac signal is analyzed at 410 and characterized as one of benign412 or treatable 414. Once the cardiac signal is characterized, as shownat 416, the method may modify “Confirmation criteria.” In this example,Confirmation criteria refers to thresholds, standards or analysis usedto determine whether a treatable cardiac arrhythmia is ongoing, in orderto confirm that therapy should be delivered once the ICSD is ready.Several examples have been discussed above.

In an illustrative example, when the cardiac signal is characterized asbenign 412, the Confirmation criteria are modified such that a greaterquantity of cardiac signal indicating a treatable arrhythmia is requiredto confirm therapy delivery. In some examples, this may includemodifying the Confirmation criteria by increasing the number ofmorphologically treatable events or short intervals are needed toconfirm therapy. In another example, rather than an event driven system,a system that analyzes blocks of time (for example, 1 second, 3 second,or other duration blocks of time, which may be separate, adjacent oroverlapping in several embodiments) for cardiac rhythm analysis may addto the number or length of such time blocks. Next, the method loops backto cardiac signal analysis 410.

Once the ICSD is ready for therapy, a transition is made to block 404,in which cardiac signal is analyzed at 420 and characterized as benign422 or treatable 424. Using these characterizations, the Confirmationcriteria are applied at 426 in order to determine whether a treatablecardiac arrhythmia is ongoing. If a treatable cardiac arrhythmia isongoing, therapy is confirmed and therefore delivered as shown at 430.Otherwise, the method loops back to analyze cardiac signal 420.

If it is determined that therapy is no longer indicated during analysisin either of block 402 or 404, such a determination is made as shown at440. If therapy is no longer indicated, then the system may performvarious methods following identification of a non-sustained treatablearrhythmia. These may include storing data related to a non-sustainedarrhythmia, annunciating potential sensing difficulties based onidentification of non-sustained arrhythmia(s), changing sensing vectors,methods or thresholds, and/or performing any cleanup or safety tasksnecessitated by the aborted therapy.

Modifying the duration of analysis or adding more events to event-basedanalysis may be considered ways of making existing analysis more or lessrigorous. In another illustrative example, the Confirmation criteria maybe modified by changing the character of analysis or adding an extralayer of analysis, rather than making existing analysis more rigorous.In some embodiments, the Confirmation criteria may use several tiers ofanalysis such as a first tier that relies onconsecutive-short-intervals, a second tier that relies on morphologyanalysis such as correlation to a Normal Sinus template, and a thirdtier that relies on analyzing an additional sensing vector to confirmtreatable arrhythmia. In a tiered Confirmation criteria embodiment, thedefault may be to rely on only a first tier of analysis for therapyconfirmation and, if cardiac signals are characterized as benign, asecond tier of analysis may be enabled, and if further cardiac signalsare characterized as benign, the third tier of analysis may be enabled.In some such embodiments, the additional tiers of analysis may be calledwith or without modifications to the first tier of analysis, or theadditional tiers may be called to replace the first tier of analysis. Insome embodiments, any added tiers may be disabled in response tocharacterization of cardiac signals as treatable.

While not shown in the above examples, the initiation of charging may beaccompanied by a blanking period of a few hundred milliseconds, ifdesired, to avoid sensing artifacts that may occur in some systems withcharge initiation. Systems may also mitigate such noise throughfiltering or avoid such noise by design of the charging circuit inaddition to or in place of blanking. As noted, not all systems/methodsrely on charging as a factor in the setting of thresholds forconfirmation.

Some examples above use a four interval average to estimate cardiacevent rate. Other sample sizes, for example from 1-20 events may be usedinstead, or even larger groups, if desired. Larger sets may providesmoother calculations, while smaller sets may be more responsive toonset of different rhythms and sudden rate changes.

U.S. Pat. No. 7,330,757, titled METHOD FOR DISCRIMINATING BETWEENVENTRICULAR AND SUPRAVENTRICULAR ARRHYTHMIAS, U.S. Pat. No. 7,248,921,titled METHOD AND DEVICES FOR PERFORMING CARDIAC WAVEFORM APPRAISAL, USPatent Application Publication Number 2009-0259271, titled METHODS ANDDEVICES FOR ACCURATELY CLASSIFYING CARDIAC ACTIVITY, and US PatentApplication Publication Number 2010-0004713, titled METHODS AND DEVICESFOR ACCURATELY CLASSIFYING CARDIAC ACTIVITY, are each incorporatedherein by reference as providing illustrative examples of implantablecardiac stimulus systems and associated methods of implant, analysis andtherapy. Reference to patents and applications is not intended aslimiting, and other methods may be used instead of or in addition to anyof these examples in other embodiments.

Various ranges for therapy delivery energy are known and are oftenpresented in terms of delivered energy. Ranges for therapy deliveryenergy may include, for example, ranges from 0.1 Joules to 35 or moreJoules for transvenous and/or epicardial systems, and sometimes higherranges for subcutaneous therapy delivery, for example, 0.1 Joules up toor in excess of 40, 65, 80 or 100 Joules. Patient anatomy and thelocation of therapy delivery electrodes can impact the energy requiredfor effective therapy. The present invention may be used in systemsdirected at treating arrhythmias occurring in one or more of theventricles and/or atria of a patient. In some examples, atrial flutteror atrial fibrillation may be treated.

Those skilled in the art will recognize that the present invention maybe manifested in a variety of forms other than the specific embodimentsdescribed and contemplated herein. Accordingly, departures in form anddetail may be made without departing from the scope and spirit of thepresent invention.

1. (canceled)
 2. A method of operation in an implantable cardiacstimulus device (ICSD) comprising a housing that contains a power sourceand operational circuitry that operates by analyzing cardiac signalsfrom implanted electrode(s) in order to determine whether a treatablearrhythmia is detected, the method comprising: the ICSD making aninitial determination that a treatable condition is occurring and inresponse: the ICSD initializing a confirmation counter storing aConfirmation Value to an Initial Confirmation Value; and the ICSDinitiating preparation for therapy delivery; after initiatingpreparation for therapy delivery and before therapy delivery, the ICSDdetecting a plurality of heart beats and determining whether the heartbeats indicate that a benign cardiac rhythm has resumed; for a pluralityof heart beats detected after the ICSD has initiated preparation fortherapy delivery, adjusting the Confirmation Value as follows: if theheart beat indicates that a benign cardiac rhythm has resumed,incrementing the Confirmation Value counter unless it is at a MaximumValue; if the heart beat does not indicate that a benign cardiac rhythmhas resumed, decrementing the Confirmation Value counter unless it is ata Minimum Value; following completion of preparations for therapydelivery, initializing a Shockable Beat counter for tracking consecutivedetected heart beats that indicate therapy is needed to an Initial ShockValue; a) after setting the Shockable Beat counter to zero, detecting aheart beat and determining whether the heart beat indicates therapy isneeded or that benign cardiac rhythm has resumed and: if the heart beatindicates therapy is needed, incrementing the Shockable Beat counter,and decrementing the Confirmation value counter unless it is already atthe Minimum Value; if the heart beat indicates benign cardiac rhythm hasresumed, resetting the Shockable Beat counter to zero and incrementingthe Confirmation Value counter unless it is already at the MaximumValue; b) after step a), comparing the Shockable Beat counter to theConfirmation Value counter and: if the Shockable Beat counter is equalto or greater than the Confirmation Value counter, the ICSD proceedingto deliver therapy to the patient; or if the Shockable Beat counter isnot equal to or greater than the Confirmation Value counter, the ICSDeither determining that therapy is no longer indicated for the patientor returning to step a).
 3. The method of claim 2 wherein preparationfor therapy delivery comprises charging an output capacitor to a therapylevel for delivery of therapy.
 4. The method of claim 2 whereinpreparation for therapy delivery comprises waiting for a timer to expireafter the initial determination and before delivering therapy.
 5. Themethod of claim 2 wherein preparation for therapy delivery compriseswaiting for a predetermined number of cardiac events to be detectedafter the initial determination and before delivering therapy.
 6. Themethod of claim 2 wherein a heart beat indicates that a benign cardiacrhythm has resumed if a cardiac rate slower than a slow-rate thresholdis calculated using data related to the heart beat.
 7. The method ofclaim 2 wherein a heart beat indicates that a benign cardiac rhythm hasresumed if it matches a stored template for a benign cardiac rhythm. 8.The method of claim 2 wherein a heart beat indicates that a benigncardiac rhythm has resumed if it shows low beat-to-beat variability incomparison to one or more adjacent-in-time heart beats.
 9. The method ofclaim 2 wherein a heart beat indicates that a benign cardiac rhythm hasresumed if it has a narrow width.
 10. The method of claim 2 wherein theInitial Shock Value is set by assessing a quantity of heart beatsdetected just prior to completion of Therapy Preparation and setting theInitial Shock Value to the number of such heartbeats that indicatetherapy is needed.
 11. The method of claim 10 wherein the quantity ofheart beats is three, and the Minimum Value for the Confirmation Valuecounter is three.
 12. An implantable cardiac stimulus device (ICSD)comprising a housing that contains a power source and operationalcircuitry that operates by analyzing cardiac signal data from implantedelectrodes in order to determine whether a treatable arrhythmia isdetected, wherein the operational circuitry is configured to perform amethod of verifying therapy delivery comprising: the ICSD making aninitial determination that a treatable condition is occurring and inresponse: the ICSD initializing a confirmation counter storing aConfirmation Value to an Initial Confirmation Value; and the ICSDinitiating preparation for therapy delivery; after initiatingpreparation for therapy delivery and before therapy delivery, the ICSDdetecting a plurality of heart beats and determining whether the heartbeats indicate that a benign cardiac rhythm has resumed; for a pluralityof heart beats detected after the ICSD has initiated preparation fortherapy delivery, adjusting the Confirmation Value as follows: if theheart beat indicates that a benign cardiac rhythm has resumed,incrementing the Confirmation Value counter unless it is at a MaximumValue; if the heart beat does not indicate that a benign cardiac rhythmhas resumed, decrementing the Confirmation Value counter unless it is ata Minimum Value; following completion of preparations for therapydelivery, initializing a Shockable Beat counter for tracking consecutivedetected heart beats that indicate therapy is needed to an Initial ShockValue; a) after setting the Shockable Beat counter to zero, detecting aheart beat and determining whether the heart beat indicates therapy isneeded or that benign cardiac rhythm has resumed and: if the heart beatindicates therapy is needed, incrementing the Shockable Beat counter,and decrementing the Confirmation value counter unless it is already atthe Minimum Value; if the heart beat indicates benign cardiac rhythm hasresumed, resetting the Shockable Beat counter to zero and incrementingthe Confirmation Value counter unless it is already at the MaximumValue; b) after step a), comparing the Shockable Beat counter to theConfirmation Value counter and: if the Shockable Beat counter is equalto or greater than the Confirmation Value counter, the ICSD proceedingto deliver therapy to the patient; or if the Shockable Beat counter isnot equal to or greater than the Confirmation Value counter, the ICSDeither determining that therapy is no longer indicated for the patientor returning to step a).
 13. The ICSD of claim 12 wherein theoperational circuitry includes: an output capacitor and a chargercoupled to the output capacitor such that the output capacitor can becharged by the charger to temporarily store therapeutic energy prior todelivering therapy; input circuitry for receiving input signals from theimplanted electrodes and amplifying and filtering the input signals; acontroller coupled to the input circuitry, output capacitor, outputcircuitry, and charger and controlling the functions thereof, thecontroller configured to determine whether a treatable arrhythmia isdetected and to perform the method of verifying therapy delivery; and abattery providing power to the operational circuitry.
 14. The ICSD ofclaim 13 wherein the operational circuitry is configured such thatpreparation for therapy comprises charging the output capacitor to atherapy level for delivery of therapy.
 15. The ICSD of claim 12 whereinthe operational circuitry is configured such that preparation fortherapy comprises waiting for a timer to expire after the initialdetermination and before delivering therapy.
 16. The ICSD of claim 12wherein the operational circuitry is configured such that preparationfor therapy comprises waiting for a predetermined number of cardiacevents to be detected after the initial determination and beforedelivering therapy.
 17. The ICSD of claim 12 wherein the operationalcircuitry is configured such that a heart beat indicates that a benigncardiac rhythm has resumed if it matches a stored template for a benigncardiac rhythm.
 18. The ICSD of claim 12 wherein the operationalcircuitry is configured such that a heart beat indicates that a benigncardiac rhythm has resumed if it shows low beat-to-beat variability incomparison to one or more adjacent-in-time heart beats.
 19. The ICSD ofclaim 12 wherein the operational circuitry is configured such that aheart beat indicates that a benign cardiac rhythm has resumed if it hasa narrow width.
 20. The ICSD of claim 12 wherein the operationalcircuitry is configured such that the Initial Shock Value is set byassessing a quantity of heart beats detected just prior to completion ofTherapy Preparation and setting the Initial Shock Value to the number ofsuch heartbeats that indicate therapy is needed.
 21. The ICSD of claim20 wherein the operational circuitry is configured such that thequantity of heart beats is three, and the Minimum Value for theConfirmation Value counter is three.